Conjoined twins are two babies that are born physically connected to each other.

Conjoined twins develop when an early embryo partially separates to form two individuals. Although two fetuses will develop from this embryo, they will remain physically connected — most often at the chest, pelvis or buttocks. Conjoined twins may also share one or more internal organs.

Most conjoined twins are stillborn or die shortly after birth. Some surviving conjoined twins can be surgically separated. The success of surgery to separate conjoined twins depends on where the twins are joined and how many and which organs are shared, as well as on the experience and skill of the surgical team.

There aren't any specific signs and symptoms that indicate a woman is carrying conjoined twins. As with other twin pregnancies, the uterus may grow more rapidly than expected, and mothers of twins may also have more fatigue, nausea and vomiting early in the pregnancy.

How twins are joined

Conjoined twins are usually classified according to where they're joined, and there are many ways that conjoined twins may be connected. Some of the more common ways include:

Joined at the chest. One of the most common of conjoined twins, thoracopagus twins are joined at the chest. They often have a shared heart and may also share one liver and upper intestine.

Joined near the bellybutton. Omphalopagus twins are joined near the bellybutton. Many omphalopagus twins share the liver, and some share the lower part of the small intestine (ileum) and colon. They generally do not, however, share a heart.

Joined at the base of the spine. Pygopagus twins are joined at the base of the spine and commonly face away from one another. Some pygopagus twins share the lower gastrointestinal tract, and a few share the genital and urinary organs.

Joined at the pelvis. Ischiopagus twins are joined at the pelvis. Many ischiopagus twins share the lower gastrointestinal tract, as well as the liver and genital and urinary tract organs. Each twin may have two legs or in some cases, one pair of legs and even a fused leg, though that's uncommon.

Joined at the head. Craniopagus twins are joined at the head. Craniopagus twins share a portion of the skull, and possibly brain tissue. This sharing may involve the cerebral cortex — the part of the brain that plays a central role in memory, language and perception.

In rare cases, twins may be asymmetrically conjoined, with one twin smaller and less fully formed than the other (parasitic twins).

Identical twins (monozygotic twins) occur when a single fertilized egg splits and develops into two individuals. Eight to 12 days after conception, the embryonic layers that will split to form monozygotic twins begin to develop into specific organs and structures. It's believed that when the embryo splits later than this — usually between 13 and 15 days after conception — separation stops before the process is complete, and the resulting twins are conjoined.

An alternative theory suggests that two separate embryos may somehow fuse together in early development.

What might cause either scenario to occur is unknown.

Because conjoined twins are so rare, and the cause isn't clear, it's unknown what might make some couples more likely to have conjoined twins. It is known, however, that conjoined twins occur more often in Latin America than they do in the United States or Europe. However, most risk factors for twins only apply to those arising from two separate eggs.

Most conjoined twins die in the womb (stillborn) or soon after birth.

Conjoined twins must be delivered by cesarean section. About 40 to 60 percent of conjoined twins are stillborn. Of conjoined twins born alive, less than half survive long enough to be candidates for separation surgery.

Conjoined twins can be diagnosed using standard ultrasound as early as the first trimester. More-detailed ultrasounds and echocardiograms can be used about halfway through pregnancy to better determine the extent of the twins' connection and functioning of their organs. False-positive results can occur before 10 weeks, however, when identical twins who share an amniotic sac (monoamniotic twins) may appear conjoined.

If an ultrasound detects conjoined twins, a magnetic resonance imaging (MRI) scan may be done. It can provide greater detail about where the conjoined twins are connected and which organs they share.

Treatment of conjoined twins depends on their unique circumstances — their health, where they're joined, and whether they share organs or other vital structures.

If you're carrying conjoined twins, you will be very closely monitored throughout your pregnancy. You will be best served by a team of doctors who work to learn as much as possible about your twins' anatomy, functional capabilities and prognosis after birth. Having this information can help your doctors form a treatment plan for your twins.

A surgical delivery (C-section) is planned ahead of time, often two to four weeks before the due date.

After your conjoined twins are born, you and your doctors must decide whether separation surgery should be attempted. An emergency separation may be needed if one of the twins dies, develops a life-threatening condition or threatens the survival of the other twin. More often, however, separation surgery is an elective procedure done two to four months after birth. Recent advances in prenatal imaging, critical care and anesthetic care have improved outcomes in separation surgery.

Many factors weigh heavily in the decision to pursue separation surgery, such as:

Do the twins share vital organs?

Are the twins healthy enough to withstand separation surgery?

What are the odds of successful separation?

What type of reconstructive surgery might be needed for each twin after successful separation?

What issues would the twins face if left conjoined?

If the circumstances are grave and separation surgery isn't possible or you decide not to pursue the surgery, comfort care — such as nutrition, fluids, human touch and pain relief — is provided as needed.

Few things are harder than to learn that your unborn child has a life-threatening condition. In the case of conjoined twins, this is doubly difficult — not only because there are two babies involved but also because the children who survive may face tremendous obstacles. Parents in these situations must grapple with very difficult decisions.

Because conjoined twins are rare, it may be difficult to find resources for conjoined twins or their families. But there are a number of organizations that support parents who have lost children or who have children with grave physical conditions. Your medical team can put you in touch with some of these groups as well as with medical social workers and counselors.

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